Estimated resident population (ERP) is the official estimate of the Australian population, which links people to a place of usual residence within Australia. Usual residence is the address at which a person considers themselves to currently live. ERP includes all people who usually live in Australia (regardless of nationality, citizenship or legal status), with the exception of foreign diplomatic personnel and their families.
ERP, or population estimates, for Australia and it's states and territories (from now referred to as states) are prepared quarterly and released around six months after the reference date in National, state and territory population.
Annual population estimates as at 30 June are then prepared for areas below the state level and released in this product. Estimates are prepared at the Statistical Area Level 2 and Local Government Area levels, according to the Australian Statistical Geography Standard (ASGS), and are aggregated or split to create estimates for other geographies. Population estimates are available in this product for Statistical Areas Levels 2 to 4, Greater Capital City Statistical Areas, Local Government Areas, Significant Urban Areas, Remoteness Areas, and Commonwealth and State Electoral Divisions.
Age and sex breakdowns of these estimates will be released on 29 August 2024 in Regional population by age and sex, 2023.
Method
ERP as at 30 June in a Census year is calculated by adjusting Census counts of Australian usual residents to account for residents temporarily overseas, people missed or counted more than once in the Census (based on the Post Enumeration Survey), and for the births, deaths and migration that happened between 30 June and Census night.
At the national and state levels, ERP is updated from the Census base every three months by taking the population estimate at the start of the quarter and adding the components of population change: natural increase (births minus deaths), net overseas migration and (in the case of state populations) net interstate migration. This is known as the component method, and uses the demographic balancing equation:
\(P_{t+1}=P_t+B−D+NOM+NIM\) where:
\(P_t\) = the estimated resident population at time point \(t\)
\(P_{t+1}\) = the estimated resident population at time point \(t+1\)
\(B\) = the number of births occurring between \(t \) and \(t+1\)
\(D\) = the number of deaths occurring between \(t\) and \(t+1\)
\(NOM\) = net overseas migration occurring between \(t\) and \(t+1\)
\(NIM\) = net interstate migration occurring between \(t\) and \(t+1\)
At the national level, net interstate migration is zero.
For Statistical Areas Level 2 (SA2s) and Local Government Areas (LGAs), population estimates are updated from the Census base annually as at 30 June also using the component method. That is, by taking the estimate at the start of the financial year and adding natural increase and net overseas and internal (moves between and within the states) migration. The components for these sub-state areas are calculated by breaking down state-level component estimates, ensuring consistency between the state and sub-state population and component data.
The components of population change (and subsequently ERP) at the LGA level are constrained to those at the SA2 level to ensure consistency between these two geographies. This is done based on the smallest possible regions where SA2 and LGA boundaries match in terms of the combined area containing resident population. For example, where one LGA aligns exactly with one SA2 or where a group of LGAs aligns with a group of SA2s, the components for these areas will generally match. Estimates at the SA2 and LGA level are ultimately constrained so that they add to the relevant state estimates.
Once the estimates are updated, they are scrutinised and validated by ABS analysts. Local knowledge, such as that advised by state governments is considered and used to adjust data for particular SA2s and LGAs. In some small areas, population change since the previous Census is assumed to be zero in the absence of reliable component data.
To provide an indication of ERP below the SA2 level, population estimates are calculated for Statistical Areas Level 1 (SA1s). For a Census year, SA2 estimates are apportioned across SA1s using usual residence Census counts. In postcensal years, the SA2 estimates are apportioned across SA1s by taking into account population change implied by Medicare and electoral roll counts at the SA1 level. Estimates for SA1s are aggregated to regions such as Remoteness Areas. For areas that cannot be built up from whole SA1s, such as electoral divisions and Postal Areas, Mesh Block Census counts are used to estimate the share of the SA1 population that resides in those areas. By these means, population estimates for areas other than those provided in this product (including SA1s) may be available on request via the ABS website.
Rebasing
In Census years, both preliminary estimates (derived from updating ERP from the previous Census) and 'rebased' population estimates (based on the current Census) are prepared. Differences between these two sets of estimates are referred to as intercensal differences. Rebased estimates of SA2 populations for previous intercensal years are derived by apportioning the intercensal difference across the five years, while constraining to state totals. Rebased 2017 to 2020 estimates were generally derived by adding one-fifth of the 2021 intercensal difference to the previous estimate of the 2017 population, two-fifths to the previous estimate of the 2018 population, and so on. Intercensal difference was apportioned based on the unrebased growth rate for some areas (e.g. newly established areas). For further information, see Methodology used in final rebased population estimates, June 2021.
Accuracy
The sub-state estimates in this product are subject to some error. Some caution should be exercised when using the estimates, especially for areas with very small population.
The accuracy of ERP can be gauged by assessing the size and direction of intercensal differences. For Australia as at 30 June 2021, the unrebased ERP over-estimated the final rebased ERP by 0.2% (62,400 people). For the states and territories, the 2021 intercensal differences ranged from -3.7% (Tasmania) to +2.1% (Northern Territory).
To assess the quality of SA2-based estimates, unrebased estimates for 2021 were converted to SA2s based on ASGS Edition 3 boundaries, and constrained to final rebased state/territory ERP. These estimates were compared with final rebased 2021 SA2 estimates. The average of the absolute values of the intercensal differences for these SA2 estimates (excluding areas with less than 1,000 people) was 4.2%.
The table below shows that intercensal differences were generally larger for very small areas, and lower for very large areas.
| Size of SA2 (people) | Number of SA2s (no.) | Average absolute intercensal difference (%) |
|---|---|---|
| 1,000 to 2,999 | 94 | 8.9 |
| 3,000 to 4,999 | 305 | 3.9 |
| 5,000 to 6,999 | 328 | 3.7 |
| 7,000 to 9,999 | 397 | 4.0 |
| 10,000 to 14,999 | 594 | 4.3 |
| 15,000 to 19,999 | 384 | 3.9 |
| 20,000 and over | 218 | 3.4 |
Status
To meet the competing demands for accuracy and timeliness, there are several versions of sub-state population estimates. Preliminary estimates are available around nine months after the reference date with revised estimates 12 months later. Rebased and final estimates are made available after each Census, when revisions are made to the estimates for all years in the previous intercensal period.
The status of annual sub-state ERP and components can change over time, from preliminary to revised to final, as new component data becomes available at the state level. With each release, ERP for the previous year is usually revised due to revisions to the component data at the state level. No updated sub-state data is used for these revisions. The table below shows the current status of sub-state ERP and the components of population change at the state level, for each year estimated out from the 2021 Census base.
| Census base | Natural Increase | Overseas migration | Interstate migration | ERP status | |
|---|---|---|---|---|---|
| June 2022 | 2021 Census | Revised - based on date of registration | Revised - based on modelled traveller behaviour | Preliminary - based on expansion factors from the 2021 Census | Revised - updated due to revised component data at state level |
| June 2023 | 2021 Census | Preliminary - based on date of registration | Preliminary - based on modelled traveller behaviour | Preliminary - based on expansion factors from the 2021 Census | Preliminary |
The sub-state components of population change for 2016-17 to 2020-21 released in previous issues of this product will not be revised, and no longer sum to rebased population change for these years.